cmd/cgo: given typedef struct S T, make C.T and C.struct_S interchangeable

For incomplete struct S, C.T and C.struct_S were interchangeable in Go 1.2
and earlier, because all incomplete types were interchangeable
(even C.struct_S1 and C.struct_S2).

CL 76450043, which fixed issue 7409, made different incomplete types
different from Go's point of view, so that they were no longer completely
interchangeable.

However, imprecision about C.T and C.struct_S - really the same
underlying C type - is the one behavior enabled by the bug that
is most likely to be depended on by existing cgo code.
Explicitly allow it, to keep that code working.

Fixes issue 7786.

LGTM=iant, r
R=golang-codereviews, iant, r
CC=golang-codereviews
https://codereview.appspot.com/98580046

1 files changed, 44 insertions, 7 deletions

diff --git a/src/cmd/cgo/gcc.go b/src/cmd/cgo/gcc.go
index c5fcdfc3d..e403f6f51 100644
--- a/src/cmd/cgo/gcc.go
+++ b/src/cmd/cgo/gcc.go
@@ -1197,12 +1197,12 @@ func (c *typeConv) Type(dtype dwarf.Type, pos token.Pos) *Type {
 		return t
 
 	case *dwarf.StructType:
-		if dt.ByteSize < 0 { // opaque struct
-			break
-		}
 		// Convert to Go struct, being careful about alignment.
 		// Have to give it a name to simulate C "struct foo" references.
 		tag := dt.StructName
+		if dt.ByteSize < 0 && tag == "" { // opaque unnamed struct - should not be possible
+			break
+		}
 		if tag == "" {
 			tag = "__" + strconv.Itoa(tagGen)
 			tagGen++
@@ -1212,6 +1212,16 @@ func (c *typeConv) Type(dtype dwarf.Type, pos token.Pos) *Type {
 		name := c.Ident("_Ctype_" + dt.Kind + "_" + tag)
 		t.Go = name // publish before recursive calls
 		goIdent[name.Name] = name
+		if dt.ByteSize < 0 {
+			// Size calculation in c.Struct/c.Opaque will die with size=-1 (unknown),
+			// so execute the basic things that the struct case would do
+			// other than try to determine a Go representation.
+			tt := *t
+			tt.C = &TypeRepr{"%s %s", []interface{}{dt.Kind, tag}}
+			tt.Go = c.Ident("struct{}")
+			typedef[name.Name] = &tt
+			break
+		}
 		switch dt.Kind {
 		case "class", "union":
 			t.Go = c.Opaque(t.Size)
@@ -1264,7 +1274,12 @@ func (c *typeConv) Type(dtype dwarf.Type, pos token.Pos) *Type {
 			tt.Go = sub.Go
 			typedef[name.Name] = &tt
 		}
-		if *godefs || *cdefs {
+
+		// If sub.Go.Name is "_Ctype_struct_foo" or "_Ctype_union_foo" or "_Ctype_class_foo",
+		// use that as the Go form for this typedef too, so that the typedef will be interchangeable
+		// with the base type.
+		// In -godefs and -cdefs mode, do this for all typedefs.
+		if isStructUnionClass(sub.Go) || *godefs || *cdefs {
 			t.Go = sub.Go
 		}
 
@@ -1327,10 +1342,19 @@ func (c *typeConv) Type(dtype dwarf.Type, pos token.Pos) *Type {
 		// be correct, so calling dtype.Size again will produce the correct value.
 		t.Size = dtype.Size()
 		if t.Size < 0 {
-			// Unsized types are [0]byte, unless they're typedefs of other types.
-			// if so, use the name of the typedef for the go name.
+			// Unsized types are [0]byte, unless they're typedefs of other types
+			// or structs with tags.
+			// if so, use the name we've already defined.
 			t.Size = 0
-			if _, ok := dtype.(*dwarf.TypedefType); !ok {
+			switch dt := dtype.(type) {
+			case *dwarf.TypedefType:
+				// ok
+			case *dwarf.StructType:
+				if dt.StructName != "" {
+					break
+				}
+				t.Go = c.Opaque(0)
+			default:
 				t.Go = c.Opaque(0)
 			}
 			if t.C.Empty() {
@@ -1347,6 +1371,19 @@ func (c *typeConv) Type(dtype dwarf.Type, pos token.Pos) *Type {
 	return t
 }
 
+// isStructUnionClass reports whether the type described by the Go syntax x
+// is a struct, union, or class with a tag.
+func isStructUnionClass(x ast.Expr) bool {
+	id, ok := x.(*ast.Ident)
+	if !ok {
+		return false
+	}
+	name := id.Name
+	return strings.HasPrefix(name, "_Ctype_struct_") ||
+		strings.HasPrefix(name, "_Ctype_union_") ||
+		strings.HasPrefix(name, "_Ctype_class_")
+}
+
 // FuncArg returns a Go type with the same memory layout as
 // dtype when used as the type of a C function argument.
 func (c *typeConv) FuncArg(dtype dwarf.Type, pos token.Pos) *Type {